1 /*
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   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
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  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
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  25 
  26 package java.lang.invoke;
  27 
  28 
  29 import java.util.*;
  30 import java.lang.invoke.LambdaForm.BasicType;
  31 import sun.invoke.util.*;
  32 import sun.misc.Unsafe;
  33 
  34 import static java.lang.invoke.MethodHandleStatics.*;
  35 import static java.lang.invoke.LambdaForm.BasicType.*;
  36 
  37 /**
  38  * A method handle is a typed, directly executable reference to an underlying method,
  39  * constructor, field, or similar low-level operation, with optional
  40  * transformations of arguments or return values.
  41  * These transformations are quite general, and include such patterns as
  42  * {@linkplain #asType conversion},
  43  * {@linkplain #bindTo insertion},
  44  * {@linkplain java.lang.invoke.MethodHandles#dropArguments deletion},
  45  * and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
  46  *
  47  * <h1>Method handle contents</h1>
  48  * Method handles are dynamically and strongly typed according to their parameter and return types.
  49  * They are not distinguished by the name or the defining class of their underlying methods.
  50  * A method handle must be invoked using a symbolic type descriptor which matches
  51  * the method handle's own {@linkplain #type type descriptor}.
  52  * <p>
  53  * Every method handle reports its type descriptor via the {@link #type type} accessor.
  54  * This type descriptor is a {@link java.lang.invoke.MethodType MethodType} object,
  55  * whose structure is a series of classes, one of which is
  56  * the return type of the method (or {@code void.class} if none).
  57  * <p>
  58  * A method handle's type controls the types of invocations it accepts,
  59  * and the kinds of transformations that apply to it.
  60  * <p>
  61  * A method handle contains a pair of special invoker methods
  62  * called {@link #invokeExact invokeExact} and {@link #invoke invoke}.
  63  * Both invoker methods provide direct access to the method handle's
  64  * underlying method, constructor, field, or other operation,
  65  * as modified by transformations of arguments and return values.
  66  * Both invokers accept calls which exactly match the method handle's own type.
  67  * The plain, inexact invoker also accepts a range of other call types.
  68  * <p>
  69  * Method handles are immutable and have no visible state.
  70  * Of course, they can be bound to underlying methods or data which exhibit state.
  71  * With respect to the Java Memory Model, any method handle will behave
  72  * as if all of its (internal) fields are final variables.  This means that any method
  73  * handle made visible to the application will always be fully formed.
  74  * This is true even if the method handle is published through a shared
  75  * variable in a data race.
  76  * <p>
  77  * Method handles cannot be subclassed by the user.
  78  * Implementations may (or may not) create internal subclasses of {@code MethodHandle}
  79  * which may be visible via the {@link java.lang.Object#getClass Object.getClass}
  80  * operation.  The programmer should not draw conclusions about a method handle
  81  * from its specific class, as the method handle class hierarchy (if any)
  82  * may change from time to time or across implementations from different vendors.
  83  *
  84  * <h1>Method handle compilation</h1>
  85  * A Java method call expression naming {@code invokeExact} or {@code invoke}
  86  * can invoke a method handle from Java source code.
  87  * From the viewpoint of source code, these methods can take any arguments
  88  * and their result can be cast to any return type.
  89  * Formally this is accomplished by giving the invoker methods
  90  * {@code Object} return types and variable arity {@code Object} arguments,
  91  * but they have an additional quality called <em>signature polymorphism</em>
  92  * which connects this freedom of invocation directly to the JVM execution stack.
  93  * <p>
  94  * As is usual with virtual methods, source-level calls to {@code invokeExact}
  95  * and {@code invoke} compile to an {@code invokevirtual} instruction.
  96  * More unusually, the compiler must record the actual argument types,
  97  * and may not perform method invocation conversions on the arguments.
  98  * Instead, it must push them on the stack according to their own unconverted types.
  99  * The method handle object itself is pushed on the stack before the arguments.
 100  * The compiler then calls the method handle with a symbolic type descriptor which
 101  * describes the argument and return types.
 102  * <p>
 103  * To issue a complete symbolic type descriptor, the compiler must also determine
 104  * the return type.  This is based on a cast on the method invocation expression,
 105  * if there is one, or else {@code Object} if the invocation is an expression
 106  * or else {@code void} if the invocation is a statement.
 107  * The cast may be to a primitive type (but not {@code void}).
 108  * <p>
 109  * As a corner case, an uncasted {@code null} argument is given
 110  * a symbolic type descriptor of {@code java.lang.Void}.
 111  * The ambiguity with the type {@code Void} is harmless, since there are no references of type
 112  * {@code Void} except the null reference.
 113  *
 114  * <h1>Method handle invocation</h1>
 115  * The first time a {@code invokevirtual} instruction is executed
 116  * it is linked, by symbolically resolving the names in the instruction
 117  * and verifying that the method call is statically legal.
 118  * This is true of calls to {@code invokeExact} and {@code invoke}.
 119  * In this case, the symbolic type descriptor emitted by the compiler is checked for
 120  * correct syntax and names it contains are resolved.
 121  * Thus, an {@code invokevirtual} instruction which invokes
 122  * a method handle will always link, as long
 123  * as the symbolic type descriptor is syntactically well-formed
 124  * and the types exist.
 125  * <p>
 126  * When the {@code invokevirtual} is executed after linking,
 127  * the receiving method handle's type is first checked by the JVM
 128  * to ensure that it matches the symbolic type descriptor.
 129  * If the type match fails, it means that the method which the
 130  * caller is invoking is not present on the individual
 131  * method handle being invoked.
 132  * <p>
 133  * In the case of {@code invokeExact}, the type descriptor of the invocation
 134  * (after resolving symbolic type names) must exactly match the method type
 135  * of the receiving method handle.
 136  * In the case of plain, inexact {@code invoke}, the resolved type descriptor
 137  * must be a valid argument to the receiver's {@link #asType asType} method.
 138  * Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
 139  * <p>
 140  * After type matching, a call to {@code invokeExact} directly
 141  * and immediately invoke the method handle's underlying method
 142  * (or other behavior, as the case may be).
 143  * <p>
 144  * A call to plain {@code invoke} works the same as a call to
 145  * {@code invokeExact}, if the symbolic type descriptor specified by the caller
 146  * exactly matches the method handle's own type.
 147  * If there is a type mismatch, {@code invoke} attempts
 148  * to adjust the type of the receiving method handle,
 149  * as if by a call to {@link #asType asType},
 150  * to obtain an exactly invokable method handle {@code M2}.
 151  * This allows a more powerful negotiation of method type
 152  * between caller and callee.
 153  * <p>
 154  * (<em>Note:</em> The adjusted method handle {@code M2} is not directly observable,
 155  * and implementations are therefore not required to materialize it.)
 156  *
 157  * <h1>Invocation checking</h1>
 158  * In typical programs, method handle type matching will usually succeed.
 159  * But if a match fails, the JVM will throw a {@link WrongMethodTypeException},
 160  * either directly (in the case of {@code invokeExact}) or indirectly as if
 161  * by a failed call to {@code asType} (in the case of {@code invoke}).
 162  * <p>
 163  * Thus, a method type mismatch which might show up as a linkage error
 164  * in a statically typed program can show up as
 165  * a dynamic {@code WrongMethodTypeException}
 166  * in a program which uses method handles.
 167  * <p>
 168  * Because method types contain "live" {@code Class} objects,
 169  * method type matching takes into account both types names and class loaders.
 170  * Thus, even if a method handle {@code M} is created in one
 171  * class loader {@code L1} and used in another {@code L2},
 172  * method handle calls are type-safe, because the caller's symbolic type
 173  * descriptor, as resolved in {@code L2},
 174  * is matched against the original callee method's symbolic type descriptor,
 175  * as resolved in {@code L1}.
 176  * The resolution in {@code L1} happens when {@code M} is created
 177  * and its type is assigned, while the resolution in {@code L2} happens
 178  * when the {@code invokevirtual} instruction is linked.
 179  * <p>
 180  * Apart from the checking of type descriptors,
 181  * a method handle's capability to call its underlying method is unrestricted.
 182  * If a method handle is formed on a non-public method by a class
 183  * that has access to that method, the resulting handle can be used
 184  * in any place by any caller who receives a reference to it.
 185  * <p>
 186  * Unlike with the Core Reflection API, where access is checked every time
 187  * a reflective method is invoked,
 188  * method handle access checking is performed
 189  * <a href="MethodHandles.Lookup.html#access">when the method handle is created</a>.
 190  * In the case of {@code ldc} (see below), access checking is performed as part of linking
 191  * the constant pool entry underlying the constant method handle.
 192  * <p>
 193  * Thus, handles to non-public methods, or to methods in non-public classes,
 194  * should generally be kept secret.
 195  * They should not be passed to untrusted code unless their use from
 196  * the untrusted code would be harmless.
 197  *
 198  * <h1>Method handle creation</h1>
 199  * Java code can create a method handle that directly accesses
 200  * any method, constructor, or field that is accessible to that code.
 201  * This is done via a reflective, capability-based API called
 202  * {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
 203  * For example, a static method handle can be obtained
 204  * from {@link java.lang.invoke.MethodHandles.Lookup#findStatic Lookup.findStatic}.
 205  * There are also conversion methods from Core Reflection API objects,
 206  * such as {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 207  * <p>
 208  * Like classes and strings, method handles that correspond to accessible
 209  * fields, methods, and constructors can also be represented directly
 210  * in a class file's constant pool as constants to be loaded by {@code ldc} bytecodes.
 211  * A new type of constant pool entry, {@code CONSTANT_MethodHandle},
 212  * refers directly to an associated {@code CONSTANT_Methodref},
 213  * {@code CONSTANT_InterfaceMethodref}, or {@code CONSTANT_Fieldref}
 214  * constant pool entry.
 215  * (For full details on method handle constants,
 216  * see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)
 217  * <p>
 218  * Method handles produced by lookups or constant loads from methods or
 219  * constructors with the variable arity modifier bit ({@code 0x0080})
 220  * have a corresponding variable arity, as if they were defined with
 221  * the help of {@link #asVarargsCollector asVarargsCollector}.
 222  * <p>
 223  * A method reference may refer either to a static or non-static method.
 224  * In the non-static case, the method handle type includes an explicit
 225  * receiver argument, prepended before any other arguments.
 226  * In the method handle's type, the initial receiver argument is typed
 227  * according to the class under which the method was initially requested.
 228  * (E.g., if a non-static method handle is obtained via {@code ldc},
 229  * the type of the receiver is the class named in the constant pool entry.)
 230  * <p>
 231  * Method handle constants are subject to the same link-time access checks
 232  * their corresponding bytecode instructions, and the {@code ldc} instruction
 233  * will throw corresponding linkage errors if the bytecode behaviors would
 234  * throw such errors.
 235  * <p>
 236  * As a corollary of this, access to protected members is restricted
 237  * to receivers only of the accessing class, or one of its subclasses,
 238  * and the accessing class must in turn be a subclass (or package sibling)
 239  * of the protected member's defining class.
 240  * If a method reference refers to a protected non-static method or field
 241  * of a class outside the current package, the receiver argument will
 242  * be narrowed to the type of the accessing class.
 243  * <p>
 244  * When a method handle to a virtual method is invoked, the method is
 245  * always looked up in the receiver (that is, the first argument).
 246  * <p>
 247  * A non-virtual method handle to a specific virtual method implementation
 248  * can also be created.  These do not perform virtual lookup based on
 249  * receiver type.  Such a method handle simulates the effect of
 250  * an {@code invokespecial} instruction to the same method.
 251  *
 252  * <h1>Usage examples</h1>
 253  * Here are some examples of usage:
 254  * <blockquote><pre>{@code
 255 Object x, y; String s; int i;
 256 MethodType mt; MethodHandle mh;
 257 MethodHandles.Lookup lookup = MethodHandles.lookup();
 258 // mt is (char,char)String
 259 mt = MethodType.methodType(String.class, char.class, char.class);
 260 mh = lookup.findVirtual(String.class, "replace", mt);
 261 s = (String) mh.invokeExact("daddy",'d','n');
 262 // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
 263 assertEquals(s, "nanny");
 264 // weakly typed invocation (using MHs.invoke)
 265 s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
 266 assertEquals(s, "savvy");
 267 // mt is (Object[])List
 268 mt = MethodType.methodType(java.util.List.class, Object[].class);
 269 mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
 270 assert(mh.isVarargsCollector());
 271 x = mh.invoke("one", "two");
 272 // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
 273 assertEquals(x, java.util.Arrays.asList("one","two"));
 274 // mt is (Object,Object,Object)Object
 275 mt = MethodType.genericMethodType(3);
 276 mh = mh.asType(mt);
 277 x = mh.invokeExact((Object)1, (Object)2, (Object)3);
 278 // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
 279 assertEquals(x, java.util.Arrays.asList(1,2,3));
 280 // mt is ()int
 281 mt = MethodType.methodType(int.class);
 282 mh = lookup.findVirtual(java.util.List.class, "size", mt);
 283 i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
 284 // invokeExact(Ljava/util/List;)I
 285 assert(i == 3);
 286 mt = MethodType.methodType(void.class, String.class);
 287 mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
 288 mh.invokeExact(System.out, "Hello, world.");
 289 // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
 290  * }</pre></blockquote>
 291  * Each of the above calls to {@code invokeExact} or plain {@code invoke}
 292  * generates a single invokevirtual instruction with
 293  * the symbolic type descriptor indicated in the following comment.
 294  * In these examples, the helper method {@code assertEquals} is assumed to
 295  * be a method which calls {@link java.util.Objects#equals(Object,Object) Objects.equals}
 296  * on its arguments, and asserts that the result is true.
 297  *
 298  * <h1>Exceptions</h1>
 299  * The methods {@code invokeExact} and {@code invoke} are declared
 300  * to throw {@link java.lang.Throwable Throwable},
 301  * which is to say that there is no static restriction on what a method handle
 302  * can throw.  Since the JVM does not distinguish between checked
 303  * and unchecked exceptions (other than by their class, of course),
 304  * there is no particular effect on bytecode shape from ascribing
 305  * checked exceptions to method handle invocations.  But in Java source
 306  * code, methods which perform method handle calls must either explicitly
 307  * throw {@code Throwable}, or else must catch all
 308  * throwables locally, rethrowing only those which are legal in the context,
 309  * and wrapping ones which are illegal.
 310  *
 311  * <h1><a name="sigpoly"></a>Signature polymorphism</h1>
 312  * The unusual compilation and linkage behavior of
 313  * {@code invokeExact} and plain {@code invoke}
 314  * is referenced by the term <em>signature polymorphism</em>.
 315  * As defined in the Java Language Specification,
 316  * a signature polymorphic method is one which can operate with
 317  * any of a wide range of call signatures and return types.
 318  * <p>
 319  * In source code, a call to a signature polymorphic method will
 320  * compile, regardless of the requested symbolic type descriptor.
 321  * As usual, the Java compiler emits an {@code invokevirtual}
 322  * instruction with the given symbolic type descriptor against the named method.
 323  * The unusual part is that the symbolic type descriptor is derived from
 324  * the actual argument and return types, not from the method declaration.
 325  * <p>
 326  * When the JVM processes bytecode containing signature polymorphic calls,
 327  * it will successfully link any such call, regardless of its symbolic type descriptor.
 328  * (In order to retain type safety, the JVM will guard such calls with suitable
 329  * dynamic type checks, as described elsewhere.)
 330  * <p>
 331  * Bytecode generators, including the compiler back end, are required to emit
 332  * untransformed symbolic type descriptors for these methods.
 333  * Tools which determine symbolic linkage are required to accept such
 334  * untransformed descriptors, without reporting linkage errors.
 335  *
 336  * <h1>Interoperation between method handles and the Core Reflection API</h1>
 337  * Using factory methods in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API,
 338  * any class member represented by a Core Reflection API object
 339  * can be converted to a behaviorally equivalent method handle.
 340  * For example, a reflective {@link java.lang.reflect.Method Method} can
 341  * be converted to a method handle using
 342  * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}.
 343  * The resulting method handles generally provide more direct and efficient
 344  * access to the underlying class members.
 345  * <p>
 346  * As a special case,
 347  * when the Core Reflection API is used to view the signature polymorphic
 348  * methods {@code invokeExact} or plain {@code invoke} in this class,
 349  * they appear as ordinary non-polymorphic methods.
 350  * Their reflective appearance, as viewed by
 351  * {@link java.lang.Class#getDeclaredMethod Class.getDeclaredMethod},
 352  * is unaffected by their special status in this API.
 353  * For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers}
 354  * will report exactly those modifier bits required for any similarly
 355  * declared method, including in this case {@code native} and {@code varargs} bits.
 356  * <p>
 357  * As with any reflected method, these methods (when reflected) may be
 358  * invoked via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.
 359  * However, such reflective calls do not result in method handle invocations.
 360  * Such a call, if passed the required argument
 361  * (a single one, of type {@code Object[]}), will ignore the argument and
 362  * will throw an {@code UnsupportedOperationException}.
 363  * <p>
 364  * Since {@code invokevirtual} instructions can natively
 365  * invoke method handles under any symbolic type descriptor, this reflective view conflicts
 366  * with the normal presentation of these methods via bytecodes.
 367  * Thus, these two native methods, when reflectively viewed by
 368  * {@code Class.getDeclaredMethod}, may be regarded as placeholders only.
 369  * <p>
 370  * In order to obtain an invoker method for a particular type descriptor,
 371  * use {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker},
 372  * or {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}.
 373  * The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual Lookup.findVirtual}
 374  * API is also able to return a method handle
 375  * to call {@code invokeExact} or plain {@code invoke},
 376  * for any specified type descriptor .
 377  *
 378  * <h1>Interoperation between method handles and Java generics</h1>
 379  * A method handle can be obtained on a method, constructor, or field
 380  * which is declared with Java generic types.
 381  * As with the Core Reflection API, the type of the method handle
 382  * will constructed from the erasure of the source-level type.
 383  * When a method handle is invoked, the types of its arguments
 384  * or the return value cast type may be generic types or type instances.
 385  * If this occurs, the compiler will replace those
 386  * types by their erasures when it constructs the symbolic type descriptor
 387  * for the {@code invokevirtual} instruction.
 388  * <p>
 389  * Method handles do not represent
 390  * their function-like types in terms of Java parameterized (generic) types,
 391  * because there are three mismatches between function-like types and parameterized
 392  * Java types.
 393  * <ul>
 394  * <li>Method types range over all possible arities,
 395  * from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a> of allowed arguments.
 396  * Generics are not variadic, and so cannot represent this.</li>
 397  * <li>Method types can specify arguments of primitive types,
 398  * which Java generic types cannot range over.</li>
 399  * <li>Higher order functions over method handles (combinators) are
 400  * often generic across a wide range of function types, including
 401  * those of multiple arities.  It is impossible to represent such
 402  * genericity with a Java type parameter.</li>
 403  * </ul>
 404  *
 405  * <h1><a name="maxarity"></a>Arity limits</h1>
 406  * The JVM imposes on all methods and constructors of any kind an absolute
 407  * limit of 255 stacked arguments.  This limit can appear more restrictive
 408  * in certain cases:
 409  * <ul>
 410  * <li>A {@code long} or {@code double} argument counts (for purposes of arity limits) as two argument slots.
 411  * <li>A non-static method consumes an extra argument for the object on which the method is called.
 412  * <li>A constructor consumes an extra argument for the object which is being constructed.
 413  * <li>Since a method handle&rsquo;s {@code invoke} method (or other signature-polymorphic method) is non-virtual,
 414  *     it consumes an extra argument for the method handle itself, in addition to any non-virtual receiver object.
 415  * </ul>
 416  * These limits imply that certain method handles cannot be created, solely because of the JVM limit on stacked arguments.
 417  * For example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created for it.
 418  * Attempts to create method handles with impossible method types lead to an {@link IllegalArgumentException}.
 419  * In particular, a method handle&rsquo;s type must not have an arity of the exact maximum 255.
 420  *
 421  * @see MethodType
 422  * @see MethodHandles
 423  * @author John Rose, JSR 292 EG
 424  */
 425 public abstract class MethodHandle {
 426     static { MethodHandleImpl.initStatics(); }
 427 
 428     /**
 429      * Internal marker interface which distinguishes (to the Java compiler)
 430      * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
 431      */
 432     @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
 433     @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
 434     @interface PolymorphicSignature { }
 435 
 436     private final MethodType type;
 437     /*private*/ final LambdaForm form;
 438     // form is not private so that invokers can easily fetch it
 439     /*private*/ MethodHandle asTypeCache;
 440     // asTypeCache is not private so that invokers can easily fetch it
 441 
 442     /**
 443      * Reports the type of this method handle.
 444      * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
 445      * @return the method handle type
 446      */
 447     public MethodType type() {
 448         return type;
 449     }
 450 
 451     /**
 452      * Package-private constructor for the method handle implementation hierarchy.
 453      * Method handle inheritance will be contained completely within
 454      * the {@code java.lang.invoke} package.
 455      */
 456     // @param type type (permanently assigned) of the new method handle
 457     /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
 458         type.getClass();  // explicit NPE
 459         form.getClass();  // explicit NPE
 460         this.type = type;
 461         this.form = form;
 462 
 463         form.prepare();  // TO DO:  Try to delay this step until just before invocation.
 464     }
 465 
 466     /**
 467      * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type match.
 468      * The symbolic type descriptor at the call site of {@code invokeExact} must
 469      * exactly match this method handle's {@link #type type}.
 470      * No conversions are allowed on arguments or return values.
 471      * <p>
 472      * When this method is observed via the Core Reflection API,
 473      * it will appear as a single native method, taking an object array and returning an object.
 474      * If this native method is invoked directly via
 475      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 476      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 477      * it will throw an {@code UnsupportedOperationException}.
 478      * @param args the signature-polymorphic parameter list, statically represented using varargs
 479      * @return the signature-polymorphic result, statically represented using {@code Object}
 480      * @throws WrongMethodTypeException if the target's type is not identical with the caller's symbolic type descriptor
 481      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 482      */
 483     public final native @PolymorphicSignature Object invokeExact(Object... args) throws Throwable;
 484 
 485     /**
 486      * Invokes the method handle, allowing any caller type descriptor,
 487      * and optionally performing conversions on arguments and return values.
 488      * <p>
 489      * If the call site's symbolic type descriptor exactly matches this method handle's {@link #type type},
 490      * the call proceeds as if by {@link #invokeExact invokeExact}.
 491      * <p>
 492      * Otherwise, the call proceeds as if this method handle were first
 493      * adjusted by calling {@link #asType asType} to adjust this method handle
 494      * to the required type, and then the call proceeds as if by
 495      * {@link #invokeExact invokeExact} on the adjusted method handle.
 496      * <p>
 497      * There is no guarantee that the {@code asType} call is actually made.
 498      * If the JVM can predict the results of making the call, it may perform
 499      * adaptations directly on the caller's arguments,
 500      * and call the target method handle according to its own exact type.
 501      * <p>
 502      * The resolved type descriptor at the call site of {@code invoke} must
 503      * be a valid argument to the receivers {@code asType} method.
 504      * In particular, the caller must specify the same argument arity
 505      * as the callee's type,
 506      * if the callee is not a {@linkplain #asVarargsCollector variable arity collector}.
 507      * <p>
 508      * When this method is observed via the Core Reflection API,
 509      * it will appear as a single native method, taking an object array and returning an object.
 510      * If this native method is invoked directly via
 511      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}, via JNI,
 512      * or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect},
 513      * it will throw an {@code UnsupportedOperationException}.
 514      * @param args the signature-polymorphic parameter list, statically represented using varargs
 515      * @return the signature-polymorphic result, statically represented using {@code Object}
 516      * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's symbolic type descriptor
 517      * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference cast fails
 518      * @throws Throwable anything thrown by the underlying method propagates unchanged through the method handle call
 519      */
 520     public final native @PolymorphicSignature Object invoke(Object... args) throws Throwable;
 521 
 522     /**
 523      * Private method for trusted invocation of a method handle respecting simplified signatures.
 524      * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
 525      * <p>
 526      * The caller signature is restricted to the following basic types:
 527      * Object, int, long, float, double, and void return.
 528      * <p>
 529      * The caller is responsible for maintaining type correctness by ensuring
 530      * that the each outgoing argument value is a member of the range of the corresponding
 531      * callee argument type.
 532      * (The caller should therefore issue appropriate casts and integer narrowing
 533      * operations on outgoing argument values.)
 534      * The caller can assume that the incoming result value is part of the range
 535      * of the callee's return type.
 536      * @param args the signature-polymorphic parameter list, statically represented using varargs
 537      * @return the signature-polymorphic result, statically represented using {@code Object}
 538      */
 539     /*non-public*/ final native @PolymorphicSignature Object invokeBasic(Object... args) throws Throwable;
 540 
 541     /**
 542      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
 543      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 544      * The trailing (not leading) argument must be a MemberName.
 545      * @param args the signature-polymorphic parameter list, statically represented using varargs
 546      * @return the signature-polymorphic result, statically represented using {@code Object}
 547      */
 548     /*non-public*/ static native @PolymorphicSignature Object linkToVirtual(Object... args) throws Throwable;
 549 
 550     /**
 551      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
 552      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 553      * The trailing (not leading) argument must be a MemberName.
 554      * @param args the signature-polymorphic parameter list, statically represented using varargs
 555      * @return the signature-polymorphic result, statically represented using {@code Object}
 556      */
 557     /*non-public*/ static native @PolymorphicSignature Object linkToStatic(Object... args) throws Throwable;
 558 
 559     /**
 560      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
 561      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 562      * The trailing (not leading) argument must be a MemberName.
 563      * @param args the signature-polymorphic parameter list, statically represented using varargs
 564      * @return the signature-polymorphic result, statically represented using {@code Object}
 565      */
 566     /*non-public*/ static native @PolymorphicSignature Object linkToSpecial(Object... args) throws Throwable;
 567 
 568     /**
 569      * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
 570      * The caller signature is restricted to basic types as with {@code invokeBasic}.
 571      * The trailing (not leading) argument must be a MemberName.
 572      * @param args the signature-polymorphic parameter list, statically represented using varargs
 573      * @return the signature-polymorphic result, statically represented using {@code Object}
 574      */
 575     /*non-public*/ static native @PolymorphicSignature Object linkToInterface(Object... args) throws Throwable;
 576 
 577     /**
 578      * Performs a variable arity invocation, passing the arguments in the given list
 579      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 580      * which mentions only the type {@code Object}, and whose arity is the length
 581      * of the argument list.
 582      * <p>
 583      * Specifically, execution proceeds as if by the following steps,
 584      * although the methods are not guaranteed to be called if the JVM
 585      * can predict their effects.
 586      * <ul>
 587      * <li>Determine the length of the argument array as {@code N}.
 588      *     For a null reference, {@code N=0}. </li>
 589      * <li>Determine the general type {@code TN} of {@code N} arguments as
 590      *     as {@code TN=MethodType.genericMethodType(N)}.</li>
 591      * <li>Force the original target method handle {@code MH0} to the
 592      *     required type, as {@code MH1 = MH0.asType(TN)}. </li>
 593      * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
 594      * <li>Invoke the type-adjusted method handle on the unpacked arguments:
 595      *     MH1.invokeExact(A0, ...). </li>
 596      * <li>Take the return value as an {@code Object} reference. </li>
 597      * </ul>
 598      * <p>
 599      * Because of the action of the {@code asType} step, the following argument
 600      * conversions are applied as necessary:
 601      * <ul>
 602      * <li>reference casting
 603      * <li>unboxing
 604      * <li>widening primitive conversions
 605      * </ul>
 606      * <p>
 607      * The result returned by the call is boxed if it is a primitive,
 608      * or forced to null if the return type is void.
 609      * <p>
 610      * This call is equivalent to the following code:
 611      * <blockquote><pre>{@code
 612      * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
 613      * Object result = invoker.invokeExact(this, arguments);
 614      * }</pre></blockquote>
 615      * <p>
 616      * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
 617      * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
 618      * It can therefore be used as a bridge between native or reflective code and method handles.
 619      *
 620      * @param arguments the arguments to pass to the target
 621      * @return the result returned by the target
 622      * @throws ClassCastException if an argument cannot be converted by reference casting
 623      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 624      * @throws Throwable anything thrown by the target method invocation
 625      * @see MethodHandles#spreadInvoker
 626      */
 627     public Object invokeWithArguments(Object... arguments) throws Throwable {
 628         int argc = arguments == null ? 0 : arguments.length;
 629         @SuppressWarnings("LocalVariableHidesMemberVariable")
 630         MethodType type = type();
 631         if (type.parameterCount() != argc || isVarargsCollector()) {
 632             // simulate invoke
 633             return asType(MethodType.genericMethodType(argc)).invokeWithArguments(arguments);
 634         }
 635         MethodHandle invoker = type.invokers().varargsInvoker();
 636         return invoker.invokeExact(this, arguments);
 637     }
 638 
 639     /**
 640      * Performs a variable arity invocation, passing the arguments in the given array
 641      * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
 642      * which mentions only the type {@code Object}, and whose arity is the length
 643      * of the argument array.
 644      * <p>
 645      * This method is also equivalent to the following code:
 646      * <blockquote><pre>{@code
 647      *   invokeWithArguments(arguments.toArray()
 648      * }</pre></blockquote>
 649      *
 650      * @param arguments the arguments to pass to the target
 651      * @return the result returned by the target
 652      * @throws NullPointerException if {@code arguments} is a null reference
 653      * @throws ClassCastException if an argument cannot be converted by reference casting
 654      * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given number of {@code Object} arguments
 655      * @throws Throwable anything thrown by the target method invocation
 656      */
 657     public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
 658         return invokeWithArguments(arguments.toArray());
 659     }
 660 
 661     /**
 662      * Produces an adapter method handle which adapts the type of the
 663      * current method handle to a new type.
 664      * The resulting method handle is guaranteed to report a type
 665      * which is equal to the desired new type.
 666      * <p>
 667      * If the original type and new type are equal, returns {@code this}.
 668      * <p>
 669      * The new method handle, when invoked, will perform the following
 670      * steps:
 671      * <ul>
 672      * <li>Convert the incoming argument list to match the original
 673      *     method handle's argument list.
 674      * <li>Invoke the original method handle on the converted argument list.
 675      * <li>Convert any result returned by the original method handle
 676      *     to the return type of new method handle.
 677      * </ul>
 678      * <p>
 679      * This method provides the crucial behavioral difference between
 680      * {@link #invokeExact invokeExact} and plain, inexact {@link #invoke invoke}.
 681      * The two methods
 682      * perform the same steps when the caller's type descriptor exactly matches
 683      * the callee's, but when the types differ, plain {@link #invoke invoke}
 684      * also calls {@code asType} (or some internal equivalent) in order
 685      * to match up the caller's and callee's types.
 686      * <p>
 687      * If the current method is a variable arity method handle
 688      * argument list conversion may involve the conversion and collection
 689      * of several arguments into an array, as
 690      * {@linkplain #asVarargsCollector described elsewhere}.
 691      * In every other case, all conversions are applied <em>pairwise</em>,
 692      * which means that each argument or return value is converted to
 693      * exactly one argument or return value (or no return value).
 694      * The applied conversions are defined by consulting the
 695      * the corresponding component types of the old and new
 696      * method handle types.
 697      * <p>
 698      * Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
 699      * or old and new return types.  Specifically, for some valid index {@code i}, let
 700      * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code =this.type().parameterType(i)}.
 701      * Or else, going the other way for return values, let
 702      * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}.
 703      * If the types are the same, the new method handle makes no change
 704      * to the corresponding argument or return value (if any).
 705      * Otherwise, one of the following conversions is applied
 706      * if possible:
 707      * <ul>
 708      * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied.
 709      *     (The types do not need to be related in any particular way.
 710      *     This is because a dynamic value of null can convert to any reference type.)
 711      * <li>If <em>T0</em> and <em>T1</em> are primitives, then a Java method invocation
 712      *     conversion (JLS 5.3) is applied, if one exists.
 713      *     (Specifically, <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.)
 714      * <li>If <em>T0</em> is a primitive and <em>T1</em> a reference,
 715      *     a Java casting conversion (JLS 5.5) is applied if one exists.
 716      *     (Specifically, the value is boxed from <em>T0</em> to its wrapper class,
 717      *     which is then widened as needed to <em>T1</em>.)
 718      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
 719      *     conversion will be applied at runtime, possibly followed
 720      *     by a Java method invocation conversion (JLS 5.3)
 721      *     on the primitive value.  (These are the primitive widening conversions.)
 722      *     <em>T0</em> must be a wrapper class or a supertype of one.
 723      *     (In the case where <em>T0</em> is Object, these are the conversions
 724      *     allowed by {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.)
 725      *     The unboxing conversion must have a possibility of success, which means that
 726      *     if <em>T0</em> is not itself a wrapper class, there must exist at least one
 727      *     wrapper class <em>TW</em> which is a subtype of <em>T0</em> and whose unboxed
 728      *     primitive value can be widened to <em>T1</em>.
 729      * <li>If the return type <em>T1</em> is marked as void, any returned value is discarded
 730      * <li>If the return type <em>T0</em> is void and <em>T1</em> a reference, a null value is introduced.
 731      * <li>If the return type <em>T0</em> is void and <em>T1</em> a primitive,
 732      *     a zero value is introduced.
 733      * </ul>
 734      * (<em>Note:</em> Both <em>T0</em> and <em>T1</em> may be regarded as static types,
 735      * because neither corresponds specifically to the <em>dynamic type</em> of any
 736      * actual argument or return value.)
 737      * <p>
 738      * The method handle conversion cannot be made if any one of the required
 739      * pairwise conversions cannot be made.
 740      * <p>
 741      * At runtime, the conversions applied to reference arguments
 742      * or return values may require additional runtime checks which can fail.
 743      * An unboxing operation may fail because the original reference is null,
 744      * causing a {@link java.lang.NullPointerException NullPointerException}.
 745      * An unboxing operation or a reference cast may also fail on a reference
 746      * to an object of the wrong type,
 747      * causing a {@link java.lang.ClassCastException ClassCastException}.
 748      * Although an unboxing operation may accept several kinds of wrappers,
 749      * if none are available, a {@code ClassCastException} will be thrown.
 750      *
 751      * @param newType the expected type of the new method handle
 752      * @return a method handle which delegates to {@code this} after performing
 753      *           any necessary argument conversions, and arranges for any
 754      *           necessary return value conversions
 755      * @throws NullPointerException if {@code newType} is a null reference
 756      * @throws WrongMethodTypeException if the conversion cannot be made
 757      * @see MethodHandles#explicitCastArguments
 758      */
 759     public MethodHandle asType(MethodType newType) {
 760         // Fast path alternative to a heavyweight {@code asType} call.
 761         // Return 'this' if the conversion will be a no-op.
 762         if (newType == type) {
 763             return this;
 764         }
 765         // Return 'this.asTypeCache' if the conversion is already memoized.
 766         MethodHandle atc = asTypeCache;
 767         if (atc != null && newType == atc.type) {
 768             return atc;
 769         }
 770         return asTypeUncached(newType);
 771     }
 772 
 773     /** Override this to change asType behavior. */
 774     /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
 775         if (!type.isConvertibleTo(newType))
 776             throw new WrongMethodTypeException("cannot convert "+this+" to "+newType);
 777         return asTypeCache = convertArguments(newType);
 778     }
 779 
 780     /**
 781      * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
 782      * and spreads its elements as positional arguments.
 783      * The new method handle adapts, as its <i>target</i>,
 784      * the current method handle.  The type of the adapter will be
 785      * the same as the type of the target, except that the final
 786      * {@code arrayLength} parameters of the target's type are replaced
 787      * by a single array parameter of type {@code arrayType}.
 788      * <p>
 789      * If the array element type differs from any of the corresponding
 790      * argument types on the original target,
 791      * the original target is adapted to take the array elements directly,
 792      * as if by a call to {@link #asType asType}.
 793      * <p>
 794      * When called, the adapter replaces a trailing array argument
 795      * by the array's elements, each as its own argument to the target.
 796      * (The order of the arguments is preserved.)
 797      * They are converted pairwise by casting and/or unboxing
 798      * to the types of the trailing parameters of the target.
 799      * Finally the target is called.
 800      * What the target eventually returns is returned unchanged by the adapter.
 801      * <p>
 802      * Before calling the target, the adapter verifies that the array
 803      * contains exactly enough elements to provide a correct argument count
 804      * to the target method handle.
 805      * (The array may also be null when zero elements are required.)
 806      * <p>
 807      * If, when the adapter is called, the supplied array argument does
 808      * not have the correct number of elements, the adapter will throw
 809      * an {@link IllegalArgumentException} instead of invoking the target.
 810      * <p>
 811      * Here are some simple examples of array-spreading method handles:
 812      * <blockquote><pre>{@code
 813 MethodHandle equals = publicLookup()
 814   .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
 815 assert( (boolean) equals.invokeExact("me", (Object)"me"));
 816 assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
 817 // spread both arguments from a 2-array:
 818 MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
 819 assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
 820 assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
 821 // try to spread from anything but a 2-array:
 822 for (int n = 0; n <= 10; n++) {
 823   Object[] badArityArgs = (n == 2 ? null : new Object[n]);
 824   try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
 825   catch (IllegalArgumentException ex) { } // OK
 826 }
 827 // spread both arguments from a String array:
 828 MethodHandle eq2s = equals.asSpreader(String[].class, 2);
 829 assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
 830 assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
 831 // spread second arguments from a 1-array:
 832 MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
 833 assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
 834 assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
 835 // spread no arguments from a 0-array or null:
 836 MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
 837 assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
 838 assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
 839 // asSpreader and asCollector are approximate inverses:
 840 for (int n = 0; n <= 2; n++) {
 841     for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
 842         MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
 843         assert( (boolean) equals2.invokeWithArguments("me", "me"));
 844         assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
 845     }
 846 }
 847 MethodHandle caToString = publicLookup()
 848   .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
 849 assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
 850 MethodHandle caString3 = caToString.asCollector(char[].class, 3);
 851 assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
 852 MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
 853 assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
 854      * }</pre></blockquote>
 855      * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract the spread arguments
 856      * @param arrayLength the number of arguments to spread from an incoming array argument
 857      * @return a new method handle which spreads its final array argument,
 858      *         before calling the original method handle
 859      * @throws NullPointerException if {@code arrayType} is a null reference
 860      * @throws IllegalArgumentException if {@code arrayType} is not an array type,
 861      *         or if target does not have at least
 862      *         {@code arrayLength} parameter types,
 863      *         or if {@code arrayLength} is negative,
 864      *         or if the resulting method handle's type would have
 865      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 866      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 867      * @see #asCollector
 868      */
 869     public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
 870         asSpreaderChecks(arrayType, arrayLength);
 871         int spreadArgPos = type.parameterCount() - arrayLength;
 872         return MethodHandleImpl.makeSpreadArguments(this, arrayType, spreadArgPos, arrayLength);
 873     }
 874 
 875     private void asSpreaderChecks(Class<?> arrayType, int arrayLength) {
 876         spreadArrayChecks(arrayType, arrayLength);
 877         int nargs = type().parameterCount();
 878         if (nargs < arrayLength || arrayLength < 0)
 879             throw newIllegalArgumentException("bad spread array length");
 880         if (arrayType != Object[].class && arrayLength != 0) {
 881             boolean sawProblem = false;
 882             Class<?> arrayElement = arrayType.getComponentType();
 883             for (int i = nargs - arrayLength; i < nargs; i++) {
 884                 if (!MethodType.canConvert(arrayElement, type().parameterType(i))) {
 885                     sawProblem = true;
 886                     break;
 887                 }
 888             }
 889             if (sawProblem) {
 890                 ArrayList<Class<?>> ptypes = new ArrayList<>(type().parameterList());
 891                 for (int i = nargs - arrayLength; i < nargs; i++) {
 892                     ptypes.set(i, arrayElement);
 893                 }
 894                 // elicit an error:
 895                 this.asType(MethodType.methodType(type().returnType(), ptypes));
 896             }
 897         }
 898     }
 899 
 900     private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
 901         Class<?> arrayElement = arrayType.getComponentType();
 902         if (arrayElement == null)
 903             throw newIllegalArgumentException("not an array type", arrayType);
 904         if ((arrayLength & 0x7F) != arrayLength) {
 905             if ((arrayLength & 0xFF) != arrayLength)
 906                 throw newIllegalArgumentException("array length is not legal", arrayLength);
 907             assert(arrayLength >= 128);
 908             if (arrayElement == long.class ||
 909                 arrayElement == double.class)
 910                 throw newIllegalArgumentException("array length is not legal for long[] or double[]", arrayLength);
 911         }
 912     }
 913 
 914     /**
 915      * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
 916      * positional arguments and collects them into an array argument.
 917      * The new method handle adapts, as its <i>target</i>,
 918      * the current method handle.  The type of the adapter will be
 919      * the same as the type of the target, except that a single trailing
 920      * parameter (usually of type {@code arrayType}) is replaced by
 921      * {@code arrayLength} parameters whose type is element type of {@code arrayType}.
 922      * <p>
 923      * If the array type differs from the final argument type on the original target,
 924      * the original target is adapted to take the array type directly,
 925      * as if by a call to {@link #asType asType}.
 926      * <p>
 927      * When called, the adapter replaces its trailing {@code arrayLength}
 928      * arguments by a single new array of type {@code arrayType}, whose elements
 929      * comprise (in order) the replaced arguments.
 930      * Finally the target is called.
 931      * What the target eventually returns is returned unchanged by the adapter.
 932      * <p>
 933      * (The array may also be a shared constant when {@code arrayLength} is zero.)
 934      * <p>
 935      * (<em>Note:</em> The {@code arrayType} is often identical to the last
 936      * parameter type of the original target.
 937      * It is an explicit argument for symmetry with {@code asSpreader}, and also
 938      * to allow the target to use a simple {@code Object} as its last parameter type.)
 939      * <p>
 940      * In order to create a collecting adapter which is not restricted to a particular
 941      * number of collected arguments, use {@link #asVarargsCollector asVarargsCollector} instead.
 942      * <p>
 943      * Here are some examples of array-collecting method handles:
 944      * <blockquote><pre>{@code
 945 MethodHandle deepToString = publicLookup()
 946   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
 947 assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
 948 MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
 949 assertEquals(methodType(String.class, Object.class), ts1.type());
 950 //assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
 951 assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
 952 // arrayType can be a subtype of Object[]
 953 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
 954 assertEquals(methodType(String.class, String.class, String.class), ts2.type());
 955 assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
 956 MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
 957 assertEquals("[]", (String) ts0.invokeExact());
 958 // collectors can be nested, Lisp-style
 959 MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class, 2);
 960 assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C", "D")));
 961 // arrayType can be any primitive array type
 962 MethodHandle bytesToString = publicLookup()
 963   .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
 964   .asCollector(byte[].class, 3);
 965 assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
 966 MethodHandle longsToString = publicLookup()
 967   .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
 968   .asCollector(long[].class, 1);
 969 assertEquals("[123]", (String) longsToString.invokeExact((long)123));
 970      * }</pre></blockquote>
 971      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
 972      * @param arrayLength the number of arguments to collect into a new array argument
 973      * @return a new method handle which collects some trailing argument
 974      *         into an array, before calling the original method handle
 975      * @throws NullPointerException if {@code arrayType} is a null reference
 976      * @throws IllegalArgumentException if {@code arrayType} is not an array type
 977      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type,
 978      *         or {@code arrayLength} is not a legal array size,
 979      *         or the resulting method handle's type would have
 980      *         <a href="MethodHandle.html#maxarity">too many parameters</a>
 981      * @throws WrongMethodTypeException if the implied {@code asType} call fails
 982      * @see #asSpreader
 983      * @see #asVarargsCollector
 984      */
 985     public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
 986         asCollectorChecks(arrayType, arrayLength);
 987         int collectArgPos = type().parameterCount()-1;
 988         MethodHandle target = this;
 989         if (arrayType != type().parameterType(collectArgPos))
 990             target = convertArguments(type().changeParameterType(collectArgPos, arrayType));
 991         MethodHandle collector = ValueConversions.varargsArray(arrayType, arrayLength);
 992         return MethodHandles.collectArguments(target, collectArgPos, collector);
 993     }
 994 
 995     // private API: return true if last param exactly matches arrayType
 996     private boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
 997         spreadArrayChecks(arrayType, arrayLength);
 998         int nargs = type().parameterCount();
 999         if (nargs != 0) {
1000             Class<?> lastParam = type().parameterType(nargs-1);
1001             if (lastParam == arrayType)  return true;
1002             if (lastParam.isAssignableFrom(arrayType))  return false;
1003         }
1004         throw newIllegalArgumentException("array type not assignable to trailing argument", this, arrayType);
1005     }
1006 
1007     /**
1008      * Makes a <em>variable arity</em> adapter which is able to accept
1009      * any number of trailing positional arguments and collect them
1010      * into an array argument.
1011      * <p>
1012      * The type and behavior of the adapter will be the same as
1013      * the type and behavior of the target, except that certain
1014      * {@code invoke} and {@code asType} requests can lead to
1015      * trailing positional arguments being collected into target's
1016      * trailing parameter.
1017      * Also, the last parameter type of the adapter will be
1018      * {@code arrayType}, even if the target has a different
1019      * last parameter type.
1020      * <p>
1021      * This transformation may return {@code this} if the method handle is
1022      * already of variable arity and its trailing parameter type
1023      * is identical to {@code arrayType}.
1024      * <p>
1025      * When called with {@link #invokeExact invokeExact}, the adapter invokes
1026      * the target with no argument changes.
1027      * (<em>Note:</em> This behavior is different from a
1028      * {@linkplain #asCollector fixed arity collector},
1029      * since it accepts a whole array of indeterminate length,
1030      * rather than a fixed number of arguments.)
1031      * <p>
1032      * When called with plain, inexact {@link #invoke invoke}, if the caller
1033      * type is the same as the adapter, the adapter invokes the target as with
1034      * {@code invokeExact}.
1035      * (This is the normal behavior for {@code invoke} when types match.)
1036      * <p>
1037      * Otherwise, if the caller and adapter arity are the same, and the
1038      * trailing parameter type of the caller is a reference type identical to
1039      * or assignable to the trailing parameter type of the adapter,
1040      * the arguments and return values are converted pairwise,
1041      * as if by {@link #asType asType} on a fixed arity
1042      * method handle.
1043      * <p>
1044      * Otherwise, the arities differ, or the adapter's trailing parameter
1045      * type is not assignable from the corresponding caller type.
1046      * In this case, the adapter replaces all trailing arguments from
1047      * the original trailing argument position onward, by
1048      * a new array of type {@code arrayType}, whose elements
1049      * comprise (in order) the replaced arguments.
1050      * <p>
1051      * The caller type must provides as least enough arguments,
1052      * and of the correct type, to satisfy the target's requirement for
1053      * positional arguments before the trailing array argument.
1054      * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
1055      * where {@code N} is the arity of the target.
1056      * Also, there must exist conversions from the incoming arguments
1057      * to the target's arguments.
1058      * As with other uses of plain {@code invoke}, if these basic
1059      * requirements are not fulfilled, a {@code WrongMethodTypeException}
1060      * may be thrown.
1061      * <p>
1062      * In all cases, what the target eventually returns is returned unchanged by the adapter.
1063      * <p>
1064      * In the final case, it is exactly as if the target method handle were
1065      * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
1066      * to the arity required by the caller type.
1067      * (As with {@code asCollector}, if the array length is zero,
1068      * a shared constant may be used instead of a new array.
1069      * If the implied call to {@code asCollector} would throw
1070      * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
1071      * the call to the variable arity adapter must throw
1072      * {@code WrongMethodTypeException}.)
1073      * <p>
1074      * The behavior of {@link #asType asType} is also specialized for
1075      * variable arity adapters, to maintain the invariant that
1076      * plain, inexact {@code invoke} is always equivalent to an {@code asType}
1077      * call to adjust the target type, followed by {@code invokeExact}.
1078      * Therefore, a variable arity adapter responds
1079      * to an {@code asType} request by building a fixed arity collector,
1080      * if and only if the adapter and requested type differ either
1081      * in arity or trailing argument type.
1082      * The resulting fixed arity collector has its type further adjusted
1083      * (if necessary) to the requested type by pairwise conversion,
1084      * as if by another application of {@code asType}.
1085      * <p>
1086      * When a method handle is obtained by executing an {@code ldc} instruction
1087      * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
1088      * as a variable arity method (with the modifier bit {@code 0x0080}),
1089      * the method handle will accept multiple arities, as if the method handle
1090      * constant were created by means of a call to {@code asVarargsCollector}.
1091      * <p>
1092      * In order to create a collecting adapter which collects a predetermined
1093      * number of arguments, and whose type reflects this predetermined number,
1094      * use {@link #asCollector asCollector} instead.
1095      * <p>
1096      * No method handle transformations produce new method handles with
1097      * variable arity, unless they are documented as doing so.
1098      * Therefore, besides {@code asVarargsCollector},
1099      * all methods in {@code MethodHandle} and {@code MethodHandles}
1100      * will return a method handle with fixed arity,
1101      * except in the cases where they are specified to return their original
1102      * operand (e.g., {@code asType} of the method handle's own type).
1103      * <p>
1104      * Calling {@code asVarargsCollector} on a method handle which is already
1105      * of variable arity will produce a method handle with the same type and behavior.
1106      * It may (or may not) return the original variable arity method handle.
1107      * <p>
1108      * Here is an example, of a list-making variable arity method handle:
1109      * <blockquote><pre>{@code
1110 MethodHandle deepToString = publicLookup()
1111   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
1112 MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
1113 assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
1114 assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
1115 assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
1116 assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
1117 // findStatic of Arrays.asList(...) produces a variable arity method handle:
1118 MethodHandle asList = publicLookup()
1119   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
1120 assertEquals(methodType(List.class, Object[].class), asList.type());
1121 assert(asList.isVarargsCollector());
1122 assertEquals("[]", asList.invoke().toString());
1123 assertEquals("[1]", asList.invoke(1).toString());
1124 assertEquals("[two, too]", asList.invoke("two", "too").toString());
1125 String[] argv = { "three", "thee", "tee" };
1126 assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
1127 assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
1128 List ls = (List) asList.invoke((Object)argv);
1129 assertEquals(1, ls.size());
1130 assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
1131      * }</pre></blockquote>
1132      * <p style="font-size:smaller;">
1133      * <em>Discussion:</em>
1134      * These rules are designed as a dynamically-typed variation
1135      * of the Java rules for variable arity methods.
1136      * In both cases, callers to a variable arity method or method handle
1137      * can either pass zero or more positional arguments, or else pass
1138      * pre-collected arrays of any length.  Users should be aware of the
1139      * special role of the final argument, and of the effect of a
1140      * type match on that final argument, which determines whether
1141      * or not a single trailing argument is interpreted as a whole
1142      * array or a single element of an array to be collected.
1143      * Note that the dynamic type of the trailing argument has no
1144      * effect on this decision, only a comparison between the symbolic
1145      * type descriptor of the call site and the type descriptor of the method handle.)
1146      *
1147      * @param arrayType often {@code Object[]}, the type of the array argument which will collect the arguments
1148      * @return a new method handle which can collect any number of trailing arguments
1149      *         into an array, before calling the original method handle
1150      * @throws NullPointerException if {@code arrayType} is a null reference
1151      * @throws IllegalArgumentException if {@code arrayType} is not an array type
1152      *         or {@code arrayType} is not assignable to this method handle's trailing parameter type
1153      * @see #asCollector
1154      * @see #isVarargsCollector
1155      * @see #asFixedArity
1156      */
1157     public MethodHandle asVarargsCollector(Class<?> arrayType) {
1158         Class<?> arrayElement = arrayType.getComponentType();
1159         boolean lastMatch = asCollectorChecks(arrayType, 0);
1160         if (isVarargsCollector() && lastMatch)
1161             return this;
1162         return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1163     }
1164 
1165     /**
1166      * Determines if this method handle
1167      * supports {@linkplain #asVarargsCollector variable arity} calls.
1168      * Such method handles arise from the following sources:
1169      * <ul>
1170      * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
1171      * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
1172      *     which resolves to a variable arity Java method or constructor
1173      * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
1174      *     which resolves to a variable arity Java method or constructor
1175      * </ul>
1176      * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke} calls
1177      * @see #asVarargsCollector
1178      * @see #asFixedArity
1179      */
1180     public boolean isVarargsCollector() {
1181         return false;
1182     }
1183 
1184     /**
1185      * Makes a <em>fixed arity</em> method handle which is otherwise
1186      * equivalent to the current method handle.
1187      * <p>
1188      * If the current method handle is not of
1189      * {@linkplain #asVarargsCollector variable arity},
1190      * the current method handle is returned.
1191      * This is true even if the current method handle
1192      * could not be a valid input to {@code asVarargsCollector}.
1193      * <p>
1194      * Otherwise, the resulting fixed-arity method handle has the same
1195      * type and behavior of the current method handle,
1196      * except that {@link #isVarargsCollector isVarargsCollector}
1197      * will be false.
1198      * The fixed-arity method handle may (or may not) be the
1199      * a previous argument to {@code asVarargsCollector}.
1200      * <p>
1201      * Here is an example, of a list-making variable arity method handle:
1202      * <blockquote><pre>{@code
1203 MethodHandle asListVar = publicLookup()
1204   .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
1205   .asVarargsCollector(Object[].class);
1206 MethodHandle asListFix = asListVar.asFixedArity();
1207 assertEquals("[1]", asListVar.invoke(1).toString());
1208 Exception caught = null;
1209 try { asListFix.invoke((Object)1); }
1210 catch (Exception ex) { caught = ex; }
1211 assert(caught instanceof ClassCastException);
1212 assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
1213 try { asListFix.invoke("two", "too"); }
1214 catch (Exception ex) { caught = ex; }
1215 assert(caught instanceof WrongMethodTypeException);
1216 Object[] argv = { "three", "thee", "tee" };
1217 assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
1218 assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
1219 assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
1220 assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
1221      * }</pre></blockquote>
1222      *
1223      * @return a new method handle which accepts only a fixed number of arguments
1224      * @see #asVarargsCollector
1225      * @see #isVarargsCollector
1226      */
1227     public MethodHandle asFixedArity() {
1228         assert(!isVarargsCollector());
1229         return this;
1230     }
1231 
1232     /**
1233      * Binds a value {@code x} to the first argument of a method handle, without invoking it.
1234      * The new method handle adapts, as its <i>target</i>,
1235      * the current method handle by binding it to the given argument.
1236      * The type of the bound handle will be
1237      * the same as the type of the target, except that a single leading
1238      * reference parameter will be omitted.
1239      * <p>
1240      * When called, the bound handle inserts the given value {@code x}
1241      * as a new leading argument to the target.  The other arguments are
1242      * also passed unchanged.
1243      * What the target eventually returns is returned unchanged by the bound handle.
1244      * <p>
1245      * The reference {@code x} must be convertible to the first parameter
1246      * type of the target.
1247      * <p>
1248      * (<em>Note:</em>  Because method handles are immutable, the target method handle
1249      * retains its original type and behavior.)
1250      * @param x  the value to bind to the first argument of the target
1251      * @return a new method handle which prepends the given value to the incoming
1252      *         argument list, before calling the original method handle
1253      * @throws IllegalArgumentException if the target does not have a
1254      *         leading parameter type that is a reference type
1255      * @throws ClassCastException if {@code x} cannot be converted
1256      *         to the leading parameter type of the target
1257      * @see MethodHandles#insertArguments
1258      */
1259     public MethodHandle bindTo(Object x) {
1260         Class<?> ptype;
1261         @SuppressWarnings("LocalVariableHidesMemberVariable")
1262         MethodType type = type();
1263         if (type.parameterCount() == 0 ||
1264             (ptype = type.parameterType(0)).isPrimitive())
1265             throw newIllegalArgumentException("no leading reference parameter", x);
1266         x = ptype.cast(x);  // throw CCE if needed
1267         return bindReceiver(x);
1268     }
1269 
1270     /**
1271      * Returns a string representation of the method handle,
1272      * starting with the string {@code "MethodHandle"} and
1273      * ending with the string representation of the method handle's type.
1274      * In other words, this method returns a string equal to the value of:
1275      * <blockquote><pre>{@code
1276      * "MethodHandle" + type().toString()
1277      * }</pre></blockquote>
1278      * <p>
1279      * (<em>Note:</em>  Future releases of this API may add further information
1280      * to the string representation.
1281      * Therefore, the present syntax should not be parsed by applications.)
1282      *
1283      * @return a string representation of the method handle
1284      */
1285     @Override
1286     public String toString() {
1287         if (DEBUG_METHOD_HANDLE_NAMES)  return debugString();
1288         return standardString();
1289     }
1290     String standardString() {
1291         return "MethodHandle"+type;
1292     }
1293     String debugString() {
1294         return standardString()+"/LF="+internalForm()+internalProperties();
1295     }
1296 
1297     //// Implementation methods.
1298     //// Sub-classes can override these default implementations.
1299     //// All these methods assume arguments are already validated.
1300 
1301     // Other transforms to do:  convert, explicitCast, permute, drop, filter, fold, GWT, catch
1302 
1303     /*non-public*/
1304     MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
1305         if (!member.isVarargs())  return this;
1306         int argc = type().parameterCount();
1307         if (argc != 0) {
1308             Class<?> arrayType = type().parameterType(argc-1);
1309             if (arrayType.isArray()) {
1310                 return MethodHandleImpl.makeVarargsCollector(this, arrayType);
1311             }
1312         }
1313         throw member.makeAccessException("cannot make variable arity", null);
1314     }
1315     /*non-public*/
1316     MethodHandle viewAsType(MethodType newType) {
1317         // No actual conversions, just a new view of the same method.
1318         return MethodHandleImpl.makePairwiseConvert(this, newType, 0);
1319     }
1320 
1321     // Decoding
1322 
1323     /*non-public*/
1324     LambdaForm internalForm() {
1325         return form;
1326     }
1327 
1328     /*non-public*/
1329     MemberName internalMemberName() {
1330         return null;  // DMH returns DMH.member
1331     }
1332 
1333     /*non-public*/
1334     Class<?> internalCallerClass() {
1335         return null;  // caller-bound MH for @CallerSensitive method returns caller
1336     }
1337 
1338     /*non-public*/
1339     MethodHandle withInternalMemberName(MemberName member) {
1340         if (member != null) {
1341             return MethodHandleImpl.makeWrappedMember(this, member);
1342         } else if (internalMemberName() == null) {
1343             // The required internaMemberName is null, and this MH (like most) doesn't have one.
1344             return this;
1345         } else {
1346             // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
1347             MethodHandle result = rebind();
1348             assert (result.internalMemberName() == null);
1349             return result;
1350         }
1351     }
1352 
1353     /*non-public*/
1354     boolean isInvokeSpecial() {
1355         return false;  // DMH.Special returns true
1356     }
1357 
1358     /*non-public*/
1359     Object internalValues() {
1360         return null;
1361     }
1362 
1363     /*non-public*/
1364     Object internalProperties() {
1365         // Override to something like "/FOO=bar"
1366         return "";
1367     }
1368 
1369     //// Method handle implementation methods.
1370     //// Sub-classes can override these default implementations.
1371     //// All these methods assume arguments are already validated.
1372 
1373     /*non-public*/ MethodHandle convertArguments(MethodType newType) {
1374         // Override this if it can be improved.
1375         return MethodHandleImpl.makePairwiseConvert(this, newType, 1);
1376     }
1377 
1378     /*non-public*/
1379     MethodHandle bindArgument(int pos, BasicType basicType, Object value) {
1380         // Override this if it can be improved.
1381         return rebind().bindArgument(pos, basicType, value);
1382     }
1383 
1384     /*non-public*/
1385     MethodHandle bindReceiver(Object receiver) {
1386         // Override this if it can be improved.
1387         return bindArgument(0, L_TYPE, receiver);
1388     }
1389 
1390     /*non-public*/
1391     MethodHandle dropArguments(MethodType srcType, int pos, int drops) {
1392         // Override this if it can be improved.
1393         return rebind().dropArguments(srcType, pos, drops);
1394     }
1395 
1396     /*non-public*/
1397     MethodHandle permuteArguments(MethodType newType, int[] reorder) {
1398         // Override this if it can be improved.
1399         return rebind().permuteArguments(newType, reorder);
1400     }
1401 
1402     /*non-public*/
1403     MethodHandle rebind() {
1404         // Bind 'this' into a new invoker, of the known class BMH.
1405         MethodType type2 = type();
1406         LambdaForm form2 = reinvokerForm(this);
1407         // form2 = lambda (bmh, arg*) { thismh = bmh[0]; invokeBasic(thismh, arg*) }
1408         return BoundMethodHandle.bindSingle(type2, form2, this);
1409     }
1410 
1411     /*non-public*/
1412     MethodHandle reinvokerTarget() {
1413         throw new InternalError("not a reinvoker MH: "+this.getClass().getName()+": "+this);
1414     }
1415 
1416     /** Create a LF which simply reinvokes a target of the given basic type.
1417      *  The target MH must override {@link #reinvokerTarget} to provide the target.
1418      */
1419     static LambdaForm reinvokerForm(MethodHandle target) {
1420         MethodType mtype = target.type().basicType();
1421         LambdaForm reinvoker = mtype.form().cachedLambdaForm(MethodTypeForm.LF_REINVOKE);
1422         if (reinvoker != null)  return reinvoker;
1423         if (mtype.parameterSlotCount() >= MethodType.MAX_MH_ARITY)
1424             return makeReinvokerForm(target.type(), target);  // cannot cache this
1425         reinvoker = makeReinvokerForm(mtype, null);
1426         return mtype.form().setCachedLambdaForm(MethodTypeForm.LF_REINVOKE, reinvoker);
1427     }
1428     private static LambdaForm makeReinvokerForm(MethodType mtype, MethodHandle customTargetOrNull) {
1429         boolean customized = (customTargetOrNull != null);
1430         MethodHandle MH_invokeBasic = customized ? null : MethodHandles.basicInvoker(mtype);
1431         final int THIS_BMH    = 0;
1432         final int ARG_BASE    = 1;
1433         final int ARG_LIMIT   = ARG_BASE + mtype.parameterCount();
1434         int nameCursor = ARG_LIMIT;
1435         final int NEXT_MH     = customized ? -1 : nameCursor++;
1436         final int REINVOKE    = nameCursor++;
1437         LambdaForm.Name[] names = LambdaForm.arguments(nameCursor - ARG_LIMIT, mtype.invokerType());
1438         Object[] targetArgs;
1439         MethodHandle targetMH;
1440         if (customized) {
1441             targetArgs = Arrays.copyOfRange(names, ARG_BASE, ARG_LIMIT, Object[].class);
1442             targetMH = customTargetOrNull;
1443         } else {
1444             names[NEXT_MH] = new LambdaForm.Name(NF_reinvokerTarget, names[THIS_BMH]);
1445             targetArgs = Arrays.copyOfRange(names, THIS_BMH, ARG_LIMIT, Object[].class);
1446             targetArgs[0] = names[NEXT_MH];  // overwrite this MH with next MH
1447             targetMH = MethodHandles.basicInvoker(mtype);
1448         }
1449         names[REINVOKE] = new LambdaForm.Name(targetMH, targetArgs);
1450         return new LambdaForm("BMH.reinvoke", ARG_LIMIT, names);
1451     }
1452 
1453     private static final LambdaForm.NamedFunction NF_reinvokerTarget;
1454     static {
1455         try {
1456             NF_reinvokerTarget = new LambdaForm.NamedFunction(MethodHandle.class
1457                 .getDeclaredMethod("reinvokerTarget"));
1458         } catch (ReflectiveOperationException ex) {
1459             throw newInternalError(ex);
1460         }
1461     }
1462 
1463     /**
1464      * Replace the old lambda form of this method handle with a new one.
1465      * The new one must be functionally equivalent to the old one.
1466      * Threads may continue running the old form indefinitely,
1467      * but it is likely that the new one will be preferred for new executions.
1468      * Use with discretion.
1469      */
1470     /*non-public*/
1471     void updateForm(LambdaForm newForm) {
1472         if (form == newForm)  return;
1473         // ISSUE: Should we have a memory fence here?
1474         UNSAFE.putObject(this, FORM_OFFSET, newForm);
1475         this.form.prepare();  // as in MethodHandle.<init>
1476     }
1477 
1478     private static final long FORM_OFFSET;
1479     static {
1480         try {
1481             FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
1482         } catch (ReflectiveOperationException ex) {
1483             throw newInternalError(ex);
1484         }
1485     }
1486 }